Add support for yaml headers

[tomzx]

Hi,

It would be great if there was support for yaml headers.

See some examples:

• http://jekyllrb.com/docs/frontmatter/
• http://rmarkdown.rstudio.com/html_document_format.html

As of now, the following

---
title: some title

---

will be rendered as an horizontal rule (hr) and title: some title as a title.

[e9t]

I second this :)